1. Field of the Invention
The present invention relates to a roller bearing unit for planetary gear used in an automatic transmission unit for vehicle to rotatably support the planetary gear on a support shaft in the transmission unit.
2. Description of the Prior Art
A typical planetary gear unit of a vehicle automatic transmission is arranged as shown in FIG. 7. In this arrangement, the planetary gear unit is rotatably provided with respect to a support shaft 1. The planetary gear unit comprises a sun gear 2, a planetary gear 6 which is rotatably supported on the support shaft 1 and a support member 3 which is mounted so as to be freely rotatable about an axis of rotation of the sun gear 2 of the planetary gear unit. The support shaft 1 is fixedly attached at its one end (the left end in FIG. 7) to the support member 3, and formed with a cylindrically convex inner race 4 on a central outer peripheral surface thereof.
The planetary gear unit further has a retaining collar 5 which is securely attached to the outer peripheral surface at the other end of the support shaft 1 (right end in the figure).
The planetary gear 6 is provided with gear teeth 7 on an outer peripheral surface thereof, and a cylindrically concave outer raceway 8 formed on an inner peripheral surface thereof. The planetary gear 6 is maintained on the support shaft 1 so as to be freely rotatable about the central outer peripheral surface of the support shaft 1 between the retaining collar 5 and the support member 3.
A double row of first and second free floating rollers 9 are provided without retainer between the inner raceway 4 and the outer raceway 8. A guide ring 10 for guiding the axially inner ends of the rollers 9 (the ends of the rollers 9 of the respective rows facing each other) is provided between the rows of rollers 9. The outer peripheral face of the guide ring 10 is provided adjacent to the outer raceway 8, so that the guide ring 10 rotates with the planetary gear 6.
At least one thrust washer 11 (two washers in the drawings) is provided at the axially inner end faces of the retaining collar 5 and the support member 3 (the faces facing each other), respectively, so that the axially outer ends of the plurality of rollers 9 are faced to the thrust washers 11.
With the above construction, the planetary gear 0 is supported so as to rotate freely with respect to the support shaft 1, and meshes with the sun gear 2 and with a ring gear 12 which is maintained concentric with the axis of the sun gear 2. The gears 2, 6 and 12 are selectively rotated or immobilized with respect to each other, a speed change, or reverse rotation of the output with respect to the input may be achieved in the planetary gear unit.
With the above mentioned planetary gear unit, the distance L between the support member 3 and the retaining collar 5, or between the pairs of thrust washers 11 when the thrust washers 11 are provided,--this is the case for all examples of the present specification, is made a little larger than the sum of the length l(el).sub.1 of the first rollers 9 in one (first row) of the rows of rollers, plus the length l(el).sub.2 of the second rollers 9 in the other row (second row) of rollers (in general l(el).sub.1 =l(el).sub.2), plus the axial length a of the guide ring 10. That is, L&gt;l(el).sub.1 +l(el).sub.2 +a. Consequently, under normal conditions, the axially outer ends of the respective rollers 9 do not rub hard against the thrust washers 11. However, if the guide ring 10, which rotates in accordance with operation of the planetary gear unit, becomes inclined, there is the possibility of the axially outer ends of the rollers 9 rubbing hard against the thrust washers 11.
More specifically, as shown in FIG. 8, during rotation of the planetary gear unit for transmission, the planetary gear 6 may become inclined as a result of axial components of force T1, T2 produced as a result of meshing between the planetary gear 6 and the sun gear 2 and ring gear 12. Consequently the end portions 9a (adjacent the thrust washers 11) of the first and second rollers 9 are driven by the outer raceway 8. As a result, the axes of the rollers 9 are skewed relative to the axis of the support shaft 1 giving rise to a force in an axial direction of the respective rollers 9. As shown by the arrow A in FIG. 8, this force at one point on the inner peripheral surface of the outer raceway 8 pushes on the first rollers 9 of one (first row) of the rows of rollers in an axially inward direction A, while at an opposite point on the inner peripheral surface of the outer raceway 8 (180 degrees around the periphery) the similar force pushes on the second rollers 9 of the other row (second row) of rollers in an axially inward direction indicated by the arrow B of the same figure.
Consequently, due to the inward pushing forces on the double row of rollers 9 at opposite positions 180 degrees around the periphery, like couple of forces, the guide ring 10 disposed between the rows of rollers 9 becomes inclined, so that the apparent axial length of the guide ring 10 is increased to b (b&gt;a).
As a result, the outer ends of the respective rollers 9 rub hard against the thrust washers 11 so that as well as producing an increase in drive (transmission) losses, there is the possibility of failure due to excessive friction and seizure under harsh conditions such as with insufficient lubrication.